As you've discovered, the Next and Previous buttons do not allow FF/RW.
Tjerk Schuringa's REMOCLONE notes indicated that he thought there might be a
way to do so by increasing the frequency of command transmissions, but I had
no success with this, and now Bjorn has confirmed this is the way things
are.

However, I structured the code in my remote to make adding addtional codes
relatively simple, so that anyone who wanted to modify Rockbox to support
additional commands would be able to add them to the remote fairly easily.

BTW, for anyone interested, a US electronics magazine (Nuts & Volts) has
published my design as an article in their May edition. Unfortunately, they
managed to garble the text I sent them a little. When the magazine has had
it's day, I intend to publish the full text on my geocities site, with PCB
layouts etc.

I gave full credit to Rockbox and Tjerk for the initial work to discover how
the Archos remote operated, so we may see some new listmembers in the next
few weeks.